OCR AS Physics

Revision Notes

2.2.1 Sources of Uncertainty

Random & Systematic Errors

  • Measurements of quantities are made with the aim of finding the true value of that quantity
  • In reality, it is impossible to obtain the true value of any quantity as there will always be a degree of uncertainty
  • The uncertainty is an estimate of the difference between a measurement reading and the true value
  • Random and systematic errors are two types of measurement errors that lead to uncertainty


Random error

  • Random errors cause unpredictable fluctuations in an instrument’s readings as a result of uncontrollable factors, such as environmental conditions
  • This affects the precision of the measurements taken, causing a wider spread of results about the mean value
  • To reduce random error:
    • Repeat measurements several times and calculate an average from them


Systematic error

  • Systematic errors arise from the use of faulty instruments used or from flaws in the experimental method
  • This type of error is repeated consistently every time the instrument is used or the method is followed, which affects the accuracy of all readings obtained
  • To reduce systematic errors:
    • Instruments should be recalibrated, or different instruments should be used
    • Corrections or adjustments should be made to the technique

Graph Accuracy Precision, downloadable AS & A Level Physics revision notes

Representing precision and accuracy on a graph


Zero error

  • This is a type of systematic error which occurs when an instrument gives a reading when the true reading is zero
    • For example, a top-ban balance that starts at 2 g instead of 0 g
  • Zero errors can be removed by taking the difference of the offset from each value
    • Eg. If a scale starts at 2 g instead of 0 g, a measurement of 50 g would actually be 50 – 2 = 48 g
    • The offset could be positive or negative

Precision & Accuracy


  • Precise measurements are ones in which there is very little spread about the mean value, in other words, how close the measured values are to each other
  • If a measurement is repeated several times, it can be described as precise when the values are very similar to, or the same as, each other
  • The precision of a measurement is reflected in the values recorded – measurements to a greater number of decimal places are said to be more precise than those to a whole number


  • A measurement is considered accurate if it is close to the true value
  • The accuracy can be increased by repeating measurements and finding a mean of the results
  • Repeating measurements also helps to identify anomalies that can be omitted from the final results


Accuracy and Precision, downloadable AS & A Level Physics revision notes

The difference between precise and accurate results

Exam Tip

It is a very common mistake to confuse precision with accuracy – measurements can be precise but not accurate if each measurement reading has the same error.

Make sure you learn that precision refers to the ability to take multiple readings with an instrument that are close to each other, whereas accuracy is the closeness of those measurements to the true value.


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